The instant invention relates to relatively low volume chemical injection pumps, and more particularly relates, in one embodiment, to low volume chemical injection pumps for use in subsea applications.
In the art and science of recovering hydrocarbons from reservoirs beneath water, such as through off shore drilling platforms and other subsea operations, it is necessary to inject treatment chemicals into the well or wellbore, the drilling fluid therein, or in hydrocarbon transmission pipelines, etc. Such treatment chemicals may include, but are not necessarily limited to, corrosion inhibitors, scale inhibitors, paraffin inhibitors, hydrate inhibitors, demulsifiers, and the like, and mixtures thereof.
The injection of treatment chemicals into these systems requires generally only low flow rates. When delivering low flow rates using positive displacement-type pumps in an atmospheric system, net positive suction head (NPSH) is often a problem. A good design for a subsea pump should try to inherently eliminate NPSH problems. Further, a major problem with positive displacement pumps, especially at high pressure, is that the check valve seats and piston/plunger packing can be inherently leaky, and cause fluid to leak through the pump, back to the suction side or back into the suction piping. Another problem with small volume, positive displacement diaphragm or plunger pumps is that they can vapor or air lock very easily. Small bubbles in the pump chamber can expand and contract with plunger movement and cavitate and stall the pump.
Further, because the location of such chemical injection pumps is by definition at the bottom of the ocean or sea, they are subjected to severe conditions and are difficult to service due to their remote location. Thus, subsea chemical injection pumps should be strong, durable, and if possible, reparable at a distance.
An object of the present invention is to provide a method and apparatus for injecting chemical into a system that is underwater or subsea.
Another object of the present invention is to provide a subsea chemical injection pump that has a minimum of moving parts.
It is yet another object of the invention is to provide a subsea chemical injection pump which can be repaired from a remote distance and/or which may continue to operate if partially disabled.
In carrying out these and other objects of the invention, there is provided, in one form, a subsea chemical injection pump having a housing comprising opposing chambers, one on either side of a central enclosure. Each chamber has parallel walls and a cross section, and the opposing chambers extend from the central enclosure on opposite sides thereof. That is, opposing chambers are lined up across the central enclosure, although the opposing chambers are not necessarily coaxial with one another. There is present in the central enclosure at least one actuator (e.g. solenoid coil), where the actuator drives an actuator rod. The actuator rod has two ends, one each extending into an opposing chamber, and a first and second plunger, one on each end of the actuator rod, where first plunger has a circumference adapted to fill and mate with the cross section of its chamber, and where second plunger has a circumference adapted to fill and mate with the cross section of its chamber. The actuator rod and plungers on either end move back and forth between maximum travel points in the opposing chambers under the influence of the actuator, alternately decreasing and increasing the volumes of the opposing chambers, respectively. A seal is preferably present on the circumference of each plunger to inhibit fluid from entering the central enclosure from the opposing chambers. An inert coolant and lubrication fluid is present in the central enclosure between the plungers. Finally, each opposing chamber contains a suction check valve and a discharge check valve therein, in a region beyond the maximum travel point of the plunger.